I. Field of the Invention
The present invention relates to a nuclear magnetic resonance diagnostic apparatus and method (referred to as "NMR" hereinafter) for producing a two-dimensional projection image (referred to as a "scanogram" hereinafter) of a specific nuclear spin density distribution present in an object to be examined, e.g., a patient (referred to as a "patient"), in two directions by using the NMR phenomenon.
II. Description of the Prior Art
To obtain a scanogram by means of a conventional diagnostic apparatus utilizing the NMR phenomenon, one-dimensional projection data (referred to as a "PD" hereinafter) has been used.
As is known, the "scanogram" is utilized to determine which portion of an object (such as a diseased portion) the NMR images are taken from, before a plurality of the formal NMR images are taken for diagnostic purpose.
To obtain the PD, a uniform static magnetic field H.sub.0 is applied to the patient along the z-axis, and a linear gradient magnetic field (referred to as a "linear gradient field") G.sub.z generated by a pair of gradient magnetic field generating coils 1A and 1B is superimposed to the static field H.sub.0 along the z-axis, as shown in FIG. 1. A specific nucleus with regard to the static field H.sub.0 resonates at an angular frequency .omega..sub.0 given by the following formula: EQU .omega..sub.0 =.gamma.H.sub.0 ( 1)
where .gamma. is a gyromagnetic ratio and is inherently determined by the individual specific nucleus. A rotating magnetic field, or an RF pulse H.sub.1 having an angular frequency .omega..sub.0 to resonate only the specific nucleus, is applied via a pair of transmitter coils 2A and 2B to the shown x-y plane of the patient P defined by the above-mentioned linear gradient field G.sub.z, so that the NMR phenomenon occurs only in a cross-sectionally sliced portion S (referred to a "sliced portion") of the patient, which is taken for tomographic images. This sliced portion is regard as a flat portion, but it has a thickness. The NMR phenomenon is observed by an NMR signal, e.g., a free induction decay signal (referred to as a "FID signal") through a pair of receiver coils or probe head coils 3A and 3B. This FID signal is subjected to Fourier transformation, thereby obtaining a signal spectrum with respect to a rotating frequency of the nuclear spin of the specific nucleus. To obtain a projection image in a predetermined direction in the x-y plane of the sliced portion S, the sliced portion S is excited to cause the NMR phenomenon. Thereafter, a magnetic field G.sub.xy having a linear gradient along the X'-axis is superimposed to the static field H.sub.0. This X'-axis is obtained by rotating the x-axis toward the y-axis through a given angle .theta.. Equivalent field lines E in the sliced portion S of the patient P then become straight, and the rotating frequency of the nuclear spin of the specific nucleus on the equivalent field line E is expressed by the above equation (1) (see FIG. 2). For illustrative convenience, it is assumed that signals D.sub.1 to D.sub.n, similar to the FID signal, are produced from the equivalent field lines E.sub.1 to E.sub.n, respectively. The amplitudes of the signals D.sub.1 to D.sub.n are proportional to the nuclear spin densities on the equivalent field lines E.sub.1 to E.sub.n, respectively extending through the sliced portions. In practice, however, the obtained FID signal is a sum of those signals D.sub.1 to D.sub.n or is a composite FID signal. When the composite FID signal is processed by Fourier transformation, a PD (one-dimensional projection image) is obtained by a projection of the sliced portion S along the x'-axis.
When successive PDs are obtained by continuously changing and projecting the sliced portion, a scanogram SG is obtained as shown in FIG. 3.
To realize a satisfactory spatial resolution of the scanogram SG in the longitudinal direction of the patient P, many projections must be performed under the conditions that the slice thickness must be very thin and any space between the adjacent sliced portions must be eliminated. However, in general, a shift of the sliced portion is performed by mechanically moving the patient P in the longitudinal direction Z. As a result, a mechanism of a patient moving device (not shown) becomes complex. Furthermore, the level of the FID signal is proportional to the quantity of excited magnetization. The thinner, the slice becomes, the smaller, the FID signal becomes. As a result, a signal to noise ratio (referred to as an "S/N" ratio hereinafter) is significantly lowered. To improve the S/N ratio, the same sliced portion must be repeatedly examined from the same projection direction. As a result, projection time is prolonged, through a PD for only one sliced portion is obtained.